1. Field of the Invention
The present invention relates to a fiber-reinforced silicon nitride ceramic having a high fracture strength and excellent toughness.
2. Description of the Related Art
A silicon nitride ceramic has a fracture strength and thermal resistance higher than those of other ceramic materials. However, in order to use the silicon nitride ceramic as mechanical parts or structural materials, brittleness inherent to the ceramic must be overcome. This brittleness can be represented by a toughness value. Although the silicon nitride ceramic has a relatively high toughness value, its toughness must be further improved in practical applications.
Extensive studies have been made to improve toughness by a composite material obtained by incorporating fibers into a silicon nitride matrix, i.e., by a fiber-reinforced ceramic.
There are two types of fibers: a whisker which is a single crystal having a size substantially equal to a silicon nitride grain; and a long fiber having a length and size much larger than those of the silicon nitride grain. Since the long fiber has a crack arrest effect and a pull-out effect as compared with the whisker, the long fiber has been regarded to be effective for improving toughness. However, the long fiber is not a single crystal unlike the whisker and often contains polycrystalline and amorphous components. C-fibers may be mixed in the cores of the long fibers, depending on a manufacturing method. For these reasons, the long fiber has heat resistance lower than that of the whisker. During the sintering of silicon nitride containing long fibers, decreases in mechanical strength and modulus of elasticity inevitably occur. In addition, good bonding strength cannot be obtained between the silicon nitride matrix and the fibers. Therefore, expected improvement of toughness has not been made. In the past studies, it was found that this problem occurred since optimal sintering additive and sintering conditions for only the silicon nitride matrix were simply applied to a composite system of long fibers. It is therefore necessary to find a sintering additive suitable for a composite ceramic system of the long fiber and the ceramic material although such an additive may not be necessarily be optimal to the matrix.